In a gas turbine engine, in particular, an aircraft gas turbine engine, compressor stalls must be avoided as much as possible. There are different types or degrees of compressor stalls. One, a full span, non-recoverable, rotating stall, brings about a major loss in thrust because a large section of the compressor simply stops pumping--to the point that the compressor cannot return to normal operation, requiring a complete engine shut-down and restart. Less serious compressor stalls appear as partial span rotating stalls near the blade tips. However, these partial span rotating stalls can lead to the undesirable compressor "surge" phenomenon, in which the complete flow transiently reverses followed by a recovery to nearly normal operation. The flow reversal and subsequent recovery "attempt" may repeat for several cycles. A compressor with good recoverability characteristics will automatically return to normal operation after very few surge cycles. The momentary thrust loss, for a portion of a second during surge and recovery, can have minimal impact on aircraft performance; even so, surge is best avoided because it can cause structural damage to vital engine components. Also, surge can sometimes transition into the highly dangerous mode of non-recoverable full span rotating stall, which results in very low thrust as well as turbine damage, mainly from overheating.
U.S. Pat. Nos. 5,165,844 and 5,165,845 and companion patent application titled SUB-IDLE STABILITY ENHANCEMENT AND ROTATING STALL RECOVERY, Ser. No. 07/859,767, filed on Mar. 30, 1992, all assigned to United Technologies Corporation, the assignee of this application, consider techniques for preventing and recovering from full span stalls. So called "full span" rotating stalls are associated with a deficiency in the engine's systems recoverability characteristics. In that regard, investigations have shown that engine recoverability is dependent on many factors including compressor design parameters, engine components, volumes, combustion stability and engine cycle.
Mass flow through the compressor stage is a function of several factors. In a full-span rotating stall, mass flow can be so small that the resulting high fuel air ratios cause internal engine temperature elevations to potentially destructive levels. If allowed to continue, this hung stall condition can lead to serious engine damage. When the compressor is in a hung stall, the pilot must shut down the engine and then try to restart it, something that is not easy to do, especially quickly.